The present invention generally relates to line control methods and systems, and more particularly to a line control method for enabling access from a mobile station to one of a plurality of base stations, and to a line control system which employs such a line control method.
In fixed communication networks on land, integrated services digital networks (ISDNs) are becoming popular because the ISDN can cope flexibly with an increasing number of subscribers and non-telephone services, such as facsimile and data transmissions.
On the other hand, mobile communication systems which provide communication services to mobile stations, such as mobile telephones and portable telephones, are also becoming popular. In addition, in order to cope with the rapidly increasing number of subscribers and to provide various services similar to those provided by the fixed communication networks, the digital system is expected to replace the existing analog system. According to the digital mobile communication system, various kinds of processing can be carried out in the transmission path with respect to the transmitting information. The transmission characteristic of the radio line is satisfactory regardless of the information source. Furthermore, the digital mobile communication system provides an improved privacy of the communication.
FIG. 1 generally shows an example of a conventional digital mobile communication system. In FIG. 1, a base station (CS.sub.1) 91.sub.1 is coupled to a public communication network (PSTN) via a two-wire subscriber line. Base stations (CS.sub.2, . . . , CS.sub.N-1) 91.sub.2, . . . , 91.sub.N-1 are respectively coupled to an integrated services digital network (ISDN) via a private branch exchange (PABX) 92. A base station (CS.sub.N) 91.sub.N is coupled to the ISDN via a network (NCC network) which is operated by a new communication company.
The base station 91.sub.1 forms a private mobile communication system which is operated by a person who installs this base station 91.sub.1. The base station 91.sub.1 provides a communication service to a portable mobile station (PS.sub.1) 93.sub.1 via a digital radio line. The base stations 91.sub.2 through 92.sub.N-1 form a private mobile communication system, which is operated by a person who installs these base stations 91.sub.2 through 92.sub.N-1 and the private branch exchange 92. The base stations 91.sub.2 through 92.sub.N-1 provide communication services to portable mobile stations (PS.sub.2 and PS.sub.3) 93.sub.2 and 93.sub.3 via digital radio lines. The base station 91.sub.N forms a public mobile communication system, which is operated by a person who installs this base station 91.sub.N. The base station 91.sub.N provides a communication service to a portable mobile station (PS.sub.4) 93.sub.4 via a digital radio line. As indicated by a dotted line in FIG. 1, each of the base stations 91.sub.1 through 91.sub.N can provide similar communication services to all portable mobile stations located within the respective service areas, and are not limited to providing the communication services to the specific portable mobile stations mentioned above.
In the digital mobile communication system having the construction described above, it is necessary to suppress the blocking probability by providing a large number of speech channels in each zone of the plurality of sub systems. For this reason, only one or two channels are allocated as common control channels for all of the base stations and mobile stations. The control channel is used to exchange control information between the base and mobile stations, depending on the calling and receiving operations of each mobile station.
Accordingly, the common control channel must not be continuously used by one particular base station or mobile station. For example, in the going control channel which is used when the base station accesses the mobile station, an intermittent transmission system is employed whereby each base station makes only one transmission for every period of 125 ms, as indicated by A1 in FIG. 2. On the other hand, in the returning control channel which is used when the mobile station accesses to the base station, a slotted aloha system is employed. Furthermore, as indicated by A2 in FIG. 2, for example, the mobile station carries out an intermittent receiving operation with the same timing as the intermittent transmitting timing of the base station 91.sub.2, which defines a zone in which this mobile station is located, and enters the control information transmitted from the base station 91.sub.2.
The base stations 91.sub.1 through 91.sub.N respectively have one to three speech channels, and provide the communication service within an area which is 30 to 50 m in radius from each installation point. For example, in a zone having a relatively large amount of traffic such as in an office which is installed with the private branch exchange 92, a lack of the speech channels occurs under a large amount of traffic. For this reason, a plurality of base stations 91.sub.2 through 91.sub.4 are installed, depending on a maximum load as shown in FIG. 3, so as to provide the communication service using the load distribution system.
In the zone in which a plurality of base stations 91.sub.2 through 91.sub.4 are provided, each mobile station enters the control information and monitors the call while intermittently receiving in the going control channel which is controlled by one (for example, the base station 91.sub.2) of the base stations 91.sub.2 through 91.sub.4.
For example, if call with respect to the mobile station 93.sub.2, which monitors the call in the above described manner, is generated in a state where all of the speech channels of the base station 91.sub.2 allocated for the call are busy, the mobile station 93.sub.2 transmits a call-in answer signal via the returning control channel as shown in FIG. 4 (a) depending on the call-in signal which is received via the going control channel. However, because the base station 91.sub.2 cannot allocate a speech channel for the call, the base station 91.sub.2 transmits a channel busy signal via the going control channel. As a result, the mobile station 932 carries out a disconnect process based on a predetermined control sequence and returns to a waiting state.
Similarly, if the mobile station 93.sub.2 calls in a state where all of the speech channels of the base station 91.sub.2 allocated for the call are busy, the mobile station 93.sub.2 transmits a call-out signal via the returning control channel, as shown in FIG. 4 (b). However, because the base station 91.sub.2 cannot allocate a speech channel, the base station 91.sub.2 transmits a channel busy signal via the going control channel. Hence, the mobile station 93.sub.2 carries out a disconnect process based on a predetermined control sequence, and transmits a busy tone signal so as to notify the operator that the call was not made in a normal manner.
Therefore, in the conventional digital mobile communication system, even if a free speech channel remains in at least one of the base stations other than the base station to which the mobile station accesses but are installed within the same zone, there is a problem in that the blocking probability of the entire system is large because the free speech channel is not allocated for the call. In addition, when the call generated from the mobile station is not completed, the mobile station must search for another base station within the same zone in synchronism with the intermittent transmitting and receiving operations which are carried out in the going control channel. For this reason, there is a problem in that it takes a long time to search for an appropriate base station with a free speech channel and to access thereto. Furthermore, there is also a problem in that the power consumption of the mobile station increases considerably when such a search operation is made to search for the appropriate base station.